CONSTRAINING THE MAASTRICHTIAN THROUGH PALEOCENE DEVELOPMENT OF THE GREAT DIVIDE AND WASHAKIE BASINS, SOUTH-CENTRAL WYOMING

From the latest Cretaceous to the early Eocene, Wyoming’s Great Divide and Washakie basins record the story of a retreating continental seaway, its related facies successions, and the complexities introduced by the initiation and unroofing of local Laramide uplifts. In an attempt to unravel key elements of this story, the stratigraphy of the Maastrichtian Lance and Paleocene Fort Union formations was correlated throughout the subsurface, resulting in basin-wide models of the Lance Formation, including the lower (unnamed) and upper Red Rim Member, and the Fort Union Formation, including the lower coal-bearing China Butte Member and upper Overland Member.

Model results, integrated with key detrital zircon samples and available biostratigraphy, yield the following conclusions: 1) The Laramide orogeny was well underway by the late Maastrichtian in this region, with no subsurface evidence of a paleo-topographic high near the Rock Springs uplift (RSU), despite the likely existence of the RSU prior to Lance time. 2) From about 69–66 Ma, the Red Rim Member shows clear signs of Laramide deformation, and its radially thinning isopach pattern and thick, massive sandstones suggest depositional proximity to an actively uplifting source to the east. Absence of the Lance Formation along the southeast flank of the RSU and onlap of Fort Union coal beds onto the Rocks Springs uplift in China Butte time suggest reinitiation of the RSU after Lance deposition but before Fort Union time. 3) The 65.3–60.9 Ma China Butte Member is a paradox: peat deposition implies low sediment supply relative to subsidence rate, but detrital zircon results indicate significant unroofing of Laramide uplifts. It is likely that subsidence in the Great Divide and Washakie basins was inherited from Lance time, and these relic low areas contained quiet swamps with sediment input that, based on detrital zircon data, was from a distal source. 4) Sands near the base of the Overland Member suggest reinitiation of Laramide uplifts at about 60.2 Ma. Subsidence rates then decreased and denudation of these uplifts continued through Fort Union time until about 55.8 Ma.

Overall, at least two discrete uplift events, one during Lance time and another during early Overland time, bracket more quiescent periods in China Butte and late Overland time.